Paragathotanais Lang 1971

Genus Paragathotanais Lang, 1971 b

Type species. Paragathotanais typicus Lang, 1971 b

Generic remarks. This genus was erected by Lang (1971b) to accommodate his new species P. t y p i c u s (by monotypy) from the Vema Expedition in the Caribbean Sea. Paragathotanais is also a cosmopolitan genus to be found in any deep-water sampling around the world (Kudinova-Pasternak 1970; Bird & Holdich 1988; Larsen 2002). The most recent key to the genus is given by Larsen (2005) but the addition of new taxa requires a modification of that key.

Although there are no problems in separating Agathotanais and Paragathotanais there seem to be some difficulties in separating the genera Paragathotanais from Paranarthrura, although the characters given by Lang (1971a) appear to be fairly easy at first glance. The diagnostic characters for Paragathotanais separating it from Paranarthrura, are (as stated by Lang): “labium as in Agathotanais; cheliped without pseudocoxa; uropod without exopod process”. However, as quite often within Tanaidacean systematics, things are not quite as simple as expected. The labium of Paragathotanais does indeed have a setulate medial seta or projection, but is hardly ‘as in Agathotanais’ as it is lacking the prominent process on the outer lateral corners found in the labium of Agathotanais (Bird & Holdich 1988; Larsen 2002, 2005). Regarding the uropod exopodal process, this is much more obvious and often acute in Paranarthrura but still, such a structure is visible in certain species of Paragathotanais (Bird & Holdich 1988; Gurreo-Kommritz 2003) albeit very reduced. As for the cheliped pseudocoxa the situation is not quite clear cut either. Larsen (2002) described P. medius as having a pseudocoxa, although later he (2005) withdrew that character, stating that this was the result of a bad dissection. It is now clear that the Paragathotanais cheliped is attached to the carapace via a large sclerite (which often comes off with the cheliped during dissection). Also Bird & Holdich (1989) noted and illustrated the differences in the degree of fusion/no fusion of ‘sclerite/pseudocoxa’ within the Paranarthrura and suggested potential taxonomic value. This sclerite has also been suggested homologous with the pseudocoxa by Larsen & Wilson (2002) and this is supported by the observations conducted in this study. Although the cheliped sclerite of Paragathotanais can be morphologically separated from the pseudocoxa of Paranarthrura, some species of Paranarthrura does not posses a pseudocoxa but rather a sclerite. Thus we are left without any clear-cut differentiating characters between Paragathotanais and Paranarthrura and these genera must be seen as gradual evolutionary line, in the direction from Paranarthrura via Paragathotanais to Agathotanais. It is here suggested that the best way to separate Paranarthrura from Paragathotanais, is by the larger and more lateral attachment of the uropods in Paranarthrura. For identification it is recommended to view the animal habitus from the dorsal view; if the uropods protrude latero-distally from the pleotelson, the species can be referred to Paranarthrura, it they are not visible then to Paragathotanais.

Kudinova-Pasternak (1970) described Paranarthrura zevinae from the Kurile-Kamchatka Trench (before the genus Paragathotanais was established), although her species does not display any uropod exopodal process. She did not illustrate the labium but did (Kudinova-Pasternak 1970:368, plate19) illustrate a structure which could be either a sclereite or a psedodocoxa. Given the considerations raised above, Paranarthrura zevinae should be placed in Paragathotanais rather than in Paranarthrura owing to the small ventrally-attached uropods.